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Astron. Astrophys. 341, 579-586 (1999)

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The nature of the circumstellar CO2 emission from M giants *

N. Ryde, K. Eriksson and B. Gustafsson

Uppsala Astronomical Observatory, Box 515, S-75120 Uppsala, Sweden

Received 21 July 1998 / Accepted 28 September 1998

Abstract

The 13-16µm region observed by the Infrared Space Observatory (ISO) of several ABG stars are discussed. We present and analyse spectra of seven M giants which show carbon dioxide features. To explain the features of the bands, we suggest they originate from two different circumstellar layers, one being a warm and high density layer close to the star, possibly making the [FORMULA] band optically thick, and the other being a large, cold and optically thin layer extending far out in the wind. This could explain the difference in temperatures of the different bands found in the analysis of the spectra and the number of molecules needed for the emission.

It is demonstrated that in spite of the bands probably not being formed in vibrational LTE, the temperatures can be estimated from the widths of the bands.

Key words: infrared: stars – stars: late-type – stars: circumstellar matter – stars: AGB and post-AGB

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.

Send offprint requests to: N. Ryde (ryde@astro.uu.se)

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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